The invention concerns a method for conditioning and homogenizing a continuously flowing glass stream in a conditioning stretch, which extends from the inlet side of a working end or a distribution channel to at least one outlet in which there is at least one cooling zone, followed by at least one homogenizing zone for the glass temperature, whereby the temperature in the working end or the distribution channel is reduced from the inlet temperature T1 to the working temperature T2, preferably for the production of molded glass articles such as containers and pressed glass articles.
Whereas the temperatures necessary for melting glass depend on the composition, on the production process and on other facts, the temperatures required for processing the glass are normally lower than the melting temperatures of the glass. Consequently the glass must be cooled between the melting and working processes. Cooling of the glass is a part of the so-called "conditioning", during which the glass is prepared for processing. The achievement of the level of thermal homogeneity necessary for the particular working process is also part of the conditioning of the glass.
Conditioning of the glass can only take place when the glass has left the actual melting unit. In the past the conditioning was mainly carried out in the so-called forehearths or feeders. Nowadays the so-called working ends or distribution channels are also used for conditioning.
Certain developments in the recent past have radically changed the situation concerning the cooling of glass. Various improvements have been made in the melting furnaces which have resulted in a significant increase in the specific melting capacity, i.e. the melting capacity related to the area of the melting zone. Consequently the temperature of the glass leaving the furnace has increased. Other melting aids, such as bubblers or bottom heating, which have the effect of increasing the glass temperature on the bottom of the melting tank, have also led to an increase in the temperature of the glass leaving the melting tank.
Continual improvements have also been made to glass processing machines, amongst other things, to increase the throughput. Whereas in the 1960's and 1970's, machines for the mass production of containers were equipped with 6, 8 or 10 stations each for two gobs, nowadays 12 to 16 stations each for two gobs or ten stations each for three or four gobs are used. The throughput capacity of individual machines has therefore been greatly increased.
As a result of the factors mentioned above, significantly more heat must now be removed from the glass after it has left the melting tank and before it is worked than in the past. The increase in the throughput of the individual machines has also reduced the residence time of the glass in those parts of the system where the glass conditioning takes place. Thus, a greater amount of heat must be removed in a shorter time. This results in the fact that the productivity of the complete production line depends to a large extent on the cooling capacity along the conditioning stretch. However, numerous technical problems must also be taken into consideration.
As a result of the relatively high viscosity of the glass, the flow of glass in working ends and forehearths, the basic form of which is normally a channel, is laminar. It is usual for a velocity profile to be established in the glass bath, in which the maximum lies on the glass surface approximately in the center of the glass bath. As the viscosity depends on the temperature of the glass, there is an interaction between the glass temperature, the heat losses and velocity of the glass. Wherever the velocity in a particular area is lower, the resulting increase in the residence time leads to higher heat losses. Thus, the temperature sinks even further, and the increased viscosity leads to an additional decrease in the velocity.
At a constant throughput, a reduction of the velocity in one area automatically leads to an increase in the velocity in other areas with higher glass temperatures. This results in a reduction of the residence time in the higher temperature areas and so reduces the effective cooling capacity. For this reason the area of the glass bath affected by a cooling system must be clearly defined, and, as far as possible, this cooling area must avoid areas in which there are low flow velocities.
Areas of low temperatures and higher viscosity produce an effective reduction in the flow cross-section, which in turn leads to an increased drop in the glass level between the melting tank and the extraction point. This can also result in production disturbances.
Furthermore, when glass of a certain composition is cooled below a specific temperature limit, which is dependent on the glass composition, crystals can be formed, a process known as "devitrification". This process can also cause significant disturbance in the production. Therefore the cooling of the glass bath to temperatures below the devitrification temperature should be avoided. As crystal formation depends on both the temperature and time, the residence time of the glass in the critical temperature range is also an important factor.
The transport of heat within the glass bath itself is almost completely by radiation, whereby the heat transport velocity depends on the glass composition. For example, the presence of ferrous iron or chromium, which are used as coloring agents in green glass, reduces the rate of heat transport in the glass bath in comparison with a colorless glass. A similar situation also occurs with amber glass. This results in a delay in the heat transport from the lower areas of the glass bath. However, the lower areas of the glass bath must be cooled. If the cooling is applied too late, then no effective cooling effect can be observed in the lower areas of the glass bath before the glass reaches the extraction point.
Numerous cooling systems for glass conditioning are known, in most of which the heat transport is primarily by radiation. This type of heat removal is advantageous because the heat is not removed directly from the glass surface, but from a layer of the glass bath, the thickness of which depends on the radiation transmission of the glass. The Stefan-Boltzmann Law is used to calculate the amount of heat transported by radiation. An important factor in this mathematical function is the temperature difference between the radiator and receiver. Applying this function to a typical case for the glass industry, the temperature of the radiator is the temperature of the glass. Therefore the temperature of the receiver determines the amount of heat which is removed.
An effective cooling system is described in European Patent 0 212 539. Openings are made in the roof of the conditioning stretch, the effective area of which can be varied by means of sliding tiles. In this way the surroundings are used as a radiation receiver, with the rate of heat transferred being determined by the effective area of the openings. Even in the worst case the temperature of the surroundings is under 100.degree. C., and is therefore much lower than the temperatures which can be reached by radiation receivers in other systems. The cooling capacity per unit area is therefore much higher with this system. However, the radiation openings create a chimney effect and therefore cause convective air movement. Such movements are difficult to control and can lead to control problems.
Even when the cooling capacity is basically sufficient, there may still be problems with cooling the lower layers of the glass bath, which retain too high a temperature, particularly in colored glasses.
U.S. Pat. No. 2,394,893 teaches the use of a rake-like, cooled stirrer to systematically stir up the contents of a working end. This solution requires a complicated apparatus, and still does not achieve homogenization of the temperature distribution, as there is not sufficient distance available for temperature equalization at the different outlets of the working end.
German Patent DE-PS 25 07 015 describes the use of water cooled stirrers in the melting tank itself, between a melting and refining section with a high temperature on the one hand and a refining zone with a lower temperature on the other hand, in order to increase the homogenization and to improve the quality of the glass. However this requires a longer melting tank, and the problems connected with further cooling and temperature homogenization before the processing of the glass are not solved.
State-of-the-art technology is mainly concerned with cooling the glass in feeders or forehearths, which are normally connected to a distribution channel.
The article by Sims, published in "GLASS INDUSTRY", November 1991, pages 8-15, entitled "INCREASED CONDITIONING TIME LEADS TO IMPROVED THERMAL HOMOGENEITY" describes the use of open radiation cooling through openings of variable area in the superstructure, as applied to forehearths. The article also discloses the application of the same principle to the superstructure of working ends, in order to apply cooling as early as possible, and thereby obtain a long residence time in the homogenizing area. However, the cooling effect in this area is limited, as the glass bath depth in working ends and distribution channels, at least in the central areas of such, is normally relatively high, as a result of the situation of the Deep Refiner and the riser connected to it. In many cases the bottom of the working end or distribution channel is inclined upwards towards the forehearths, so that particularly difficult conditions for the cooling of the glass exist, especially in the deeper parts of the working end or distribution channel. This is the result of the fact that the energy can only or must be removed from the glass by radiation, whereby the radiation is increasingly inhibited as the glass depth increases, as the energy is reabsorbed by the upper layers of the glass.
The conditions are particularly difficult in the case of amber or green glasses, which absorb a significant proportion of the longer wavelength radiation. In "Glass Furnaces" (German--"Glasschmelzwannen"), published by the Springer Publishing Company in 1984, Trier, shows in a diagram on pages 211 and 212 that the radiation transmission of amber and green glasses at a temperature of 1300.degree. is only approximately 15-25% of the transmission of white flint glass (for example for tableware or window glass). Increasingly poor cooling conditions therefore exist with both increasing glass bath depth and increasing glass color. This leads to increasingly large temperature differences between the glass surface and the bottom parts of the glass bath, so that the throughput current results in a slow moving cooler surface layer, with a fast moving hotter layer underneath. Particularly long homogenization zones are then necessary to compensate for these conditions, whereby such zones require significant amounts of energy. Furthermore, the space requirement cannot be easily solved. As already indicated, these problems also increase in severity by a factor of 4-6 as the glass color becomes darker.
The problem of cooling in the deeper areas of the glass bath could conceivably be solved by reducing the glass bath depth in the complete working end or distribution channel. However, this solution would lead to the establishment of a glass level loss as a result of the temperature dependance of the glass viscosity and the typical flow pattern which occurs in highly viscous liquids, whereby the extent of the glass level loss would increase with increasing throughput. High throughput levels are exactly what is required for modern glass production units. However, a significant glass level loss must be avoided in working ends or distribution channels, as this would make it impossible to apply the same production parameters at each outlet.